Self-heating Mechanism Research Articles

Self-Heating and Equivalent Channel Temperature in Short Gate Length GaN HEMTs

In this paper, we study the self-heating mechanism and its impact on electrical performance of short gate length GaN high electron mobility transistors (HEMTs) based on electrothermal TCAD simulations. We propose an equivalent channel temperature to quantify the current degradation due to self-heating and also resolve the discrepancies between temperature measurements through electrical methods and thermal methods in the literature. We then explain the equivalent channel temperature’s behavior using the temperature- and field-dependent electron transport theory for short gate length HEMTs. The implications and guidelines to the various aspects of device design are also discussed.

Infrared thermographic analysis of notch effects on tensile behavior of 30CrMnSiA steel

Using infrared thermography, three types of specimens are tested to evaluate the effects of notches on the tensile strength and self-heating phenomenon. In the process of tensile tests, the local strain and temperature evolution are measured to study the deformation process. The temperature signals are divided into four phases with different self-heating mechanisms. The results show that the notch strengthening effect increases the tensile strength of the specimen and reduces the toughness and ductility; the notched specimen exhibits localized and higher rate of strain and temperature increase due to the local stress concentration. The fracture surface analysis illustrates the differences of micro fracture features between the unnotched and notched specimens.

Assessment of Self-Heating Effects Under Lateral Scaling of GaN HEMTs

The impact on self-heating mechanisms observed in GaN HEMTs fabricated on Si substrates is studied by means of a cellular Monte Carlo particle-based device simulator. Within this framework, the thermal effects are included through an energy-balance equation for phonons allowing for self-consistently coupling the charge and heat transport. First, the advanced electrothermal model of an experimental device is developed and calibrated to measured dc characteristics, showing an accurate description throughout the ${I}_{\textsf {DS}}$ ( ${V}_{\textsf {GS}}-{V}_{\textsf {DS}}$ ) space, as a result of capturing the temperature dependence of the scattering processes that modify the charge transport. Then, the model is used to assess the effect of lateral scaling, i.e., reducing the source-to-gate ${L}_{\textsf {SG}}$ and gate-to-drain ${L}_{\textsf {GD}}$ dimensions, in terms of detailed temperature maps obtained for the acoustic and optical phonon modes as well as the electric field and carrier velocity profiles. It is found that the hot spot in the channel is not located at the peak electric field as predicted by previous methods, but instead, it is shifted toward the drain up to 32 nm. Furthermore, it is shown that, while scaled devices offer improved dc and small-signal ac performance, they are subjected to temperatures up to 15% higher in the channel as compared to the original nonscaled device when dissipating the same dc power, and the temperature distribution throughout the device shows a strong correlation with the scaled layout.

Evidence for Precursors of the Coronal Hole Jets in Solar Bright Points

Abstract A set of 23 observations of coronal jet events that occurred in coronal bright points has been analyzed. The focus was on the temporal evolution of the mean brightness before and during coronal jet events. In the absolute majority of the cases either single or recurrent coronal jets (CJs) were preceded by slight precursor disturbances observed in the mean intensity curves. The key conclusion is that we were able to detect quasi-periodical oscillations with characteristic periods from sub-minute up to 3–4 minute values in the bright point brightness that precedes the jets. Our basic claim is that along with the conventionally accepted scenario of bright-point evolution through new magnetic flux emergence and its reconnection with the initial structure of the bright point and the coronal hole, certain magnetohydrodynamic (MHD) oscillatory and wavelike motions can be excited and these can take an important place in the observed dynamics. These quasi-oscillatory phenomena might play the role of links between different epochs of the coronal jet ignition and evolution. They can be an indication of the MHD wave excitation processes due to the system entropy variations, density variations, or shear flows. It is very likely a sharp outflow velocity transverse gradients at the edges between the open and closed field line regions. We suppose that magnetic reconnections can be the source of MHD waves due to impulsive generation or rapid temperature variations, and shear flow driven nonmodel MHD wave evolution (self-heating and/or overreflection mechanisms).

Combined dehydrogenation and hydrogen-based power generation

An energy production from the combination of dehydrogenation and combined cycle power generation is proposed. The delivered system is established from three main modules: dehydrogenation, combustion, and combined cycle. The heat in the system is circulated thoroughly to enhance the energy efficiency due to optimum energy recovery. The Pt/Al2O3 catalyst is applied in the dehydrogenation module due to superior activity to accelerate the dehydrogenation of MCH. The toluene emitted from the MCH is recirculated to the hydrogenation plant, while the hydrogen is further utilized as the fuel in the combustion. Although the high-temperature condition is necessary to perform high yield dehydrogenation, the proposed system is capable of carrying out self-heating mechanism with no external heat. With the optimum configuration, the delivered system can produce 100.0 MW of electricity from 100 t/h of MCH, with 50.19% of energy efficiency.

Quantum chemistry calculation of reaction pathways of carboxyl groups during coal self-heating

During coal self-heating, reactions of carboxyl groups feature in the evolution of the spontaneous combustion of coal. However, their elementary reaction pathways during this process still have not been revealed. This paper selected the Ar–CH2–COOH as a typical carboxyl group containing structure for the analysis of the reaction pathways and enhancement effect on the coal self-heating process by quantum chemistry calculations. The results indicate that the hydrogen atoms in carboxyl groups are the active sites, which undergo the oxidation process and self-reaction process during coal self-heating. They both have two elementary reactions, namely (i) the hydrogen abstraction of –COOH by oxygen and the decarboxylation of the –COO· free radical and (ii) the hydrogen abstraction of –COOH and its pyrolysis. The total enthalpy change and activation energy of the oxidation process are 76.93 kJ/mol and 127.85 kJ/mol, respectively, which indicate that this process is endothermic and will occur at medium temperatures. For the hydrogen abstraction of –COOH by hydrocarbon free radicals, the thermal parameters are 53.53 kJ/mol and 56.13 kJ/mol, respectively, which has the same thermodynamic properties as the oxidation process. However, for the pyrolysis, the thermal parameters are –42.53 kJ/mol and 493.68 kJ/mol, respectively, and is thus exothermic and would not occur until the coal reaches high temperatures. They affect heat accumulation greatly, generate carbon dioxide, and provide new active centers for enhancing the coal self-heating process. The results would be helpful for further understanding of the coal self-heating mechanism.

Transient deformation and heat generation of solid polyurethane under impact compression

This investigation examines the transient deformation and heat generation of a solid polyurethane subjected to dynamic compression. A special method is presented to prepare the solid polyurethane from raw materials which are commonly used to make polyurethane foams. Testing methods including infrared spectrum, differential scanning calorimetry, quasi-static and dynamic compression were applied to study the basic physical properties of the solid polyurethane. High-speed optical and infrared imaging systems are used to obtain visual and thermo-graphic images during impact tests. Under quasi-static compression, the solid polyurethane presents a good performance in toughness. This is confirmed by its Poisson's ratio. Under impact compression, the adiabatic heat generation are identified statistically. Temperature distribution confirms the fact of transient heat generation in specimens. Adiabatic self-heating mechanism provides a consideration to understand the negative strain-rate effect and post-yield softening effect found in the solid polyurethane. Mechanical properties including quasi-static and dynamic responses are related with the composition of molecular and structure of polymer.

Start-to-end simulation of the shot-noise driven microbunching instability experiment at the Linac Coherent Light Source

The shot-noise driven microbunching instability can significantly degrade electron beam quality in x-ray free electron laser light sources. Experiments were carried out at the Linac Coherent Light Source (LCLS) to study this instability. In this paper, we present start-to-end simulations of the shot-noise driven microbunching instability experiment at the LCLS using the real number of electrons. The simulation results reproduce the measurements quite well. A microbunching self-heating mechanism is also illustrated in the simulation, which helps explain the experimental observation.

Reaction pathways of hydroxyl groups during coal spontaneous combustion

Hydroxyl groups are one of the key factors for the development of coal self-heating, although their detailed reaction pathways are still unclear. This study investigated the reaction pathways in coal self-heating by the method of quantum chemistry calculation. The Ar–CH2–CH(CH3)–OH was selected as a typical structure unit for the calculation. The results indicate that the hydrogen atoms in hydroxyl groups and R3–CH are the active sites. For the hydrogen atoms in hydroxyl groups, they are directly abstracted by oxygen. For hydrogen atoms in R3–CH, they are abstracted by oxygen at first and generate peroxy-hydroxyl free radicals, which abstract the hydrogen atoms in hydroxyl groups later. The reaction of R3–CH contains three elementary reactions, i.e., the hydrogen abstraction of R3–CH by oxygen, the conjugation reaction between the R3C■ and oxygen atom, and the hydrogen abstraction of –OH by hydroxyl free radicals. Then, the microstructure parameters, IRC pathways, and reaction dynamic parameters were respectively analyzed for the four reactions. For the hydrogen abstraction of –OH by oxygen, the enthalpy change and activation energy are 137.63 and 334.44 kJ/mol, respectively, which will occur at medium temperatures and the corresponding heat effect is great. For the reaction of R3–CH, the enthalpy change and the activation energy are −3.45 and 55.79 kJ/mol, respectively, which will occur at low temperatures while the corresponding heat influence is weak. They both affect heat accumulation and provide new active centers for enhancing the coal self-heating process. The results would be helpful for further understanding of the coal self-heating mechanism.

Lithium-ion battery structure that self-heats at low temperatures.

Lithium-ion batteries suffer severe power loss at temperatures below zero degrees Celsius, limiting their use in applications such as electric cars in cold climates and high-altitude drones. The practical consequences of such power loss are the need for larger, more expensive battery packs to perform engine cold cranking, slow charging in cold weather, restricted regenerative braking, and reduction of vehicle cruise range by as much as 40 per cent. Previous attempts to improve the low-temperature performance of lithium-ion batteries have focused on developing additives to improve the low-temperature behaviour of electrolytes, and on externally heating and insulating the cells. Here we report a lithium-ion battery structure, the 'all-climate battery' cell, that heats itself up from below zero degrees Celsius without requiring external heating devices or electrolyte additives. The self-heating mechanism creates an electrochemical interface that is favourable for high discharge/charge power. We show that the internal warm-up of such a cell to zero degrees Celsius occurs within 20 seconds at minus 20 degrees Celsius and within 30 seconds at minus 30 degrees Celsius, consuming only 3.8 per cent and 5.5 per cent of cell capacity, respectively. The self-heated all-climate battery cell yields a discharge/regeneration power of 1,061/1,425 watts per kilogram at a 50 per cent state of charge and at minus 30 degrees Celsius, delivering 6.4-12.3 times the power of state-of-the-art lithium-ion cells. We expect the all-climate battery to enable engine stop-start technology capable of saving 5-10 per cent of the fuel for 80 million new vehicles manufactured every year. Given that only a small fraction of the battery energy is used for self-heating, we envisage that the all-climate battery cell may also prove useful for plug-in electric vehicles, robotics and space exploration applications.

Simulation of coal self-heating processes in underground methane-rich coal seams

In this work, we revised our previous fully coupled model of coal deformation, methane diffusion from matrix and compositional gas flow in fractures, and thermal transport to investigate coal self-heating processes in underground methane-rich coal seams. The coal self-heating in underground coal seams involves a chain of physico-chemical reactions, which are linked together through compositional gas flow and diffusion, reaction kinetics, energy transport and coal deformation mechanisms. Although coal–oxygen reactions have been comprehensively investigated, fewer studies consider the collective impacts of methane-desorption induced coal deformation and compositional gas concentration changes on the methane-rich coal self-heating. We included these self-heating mechanisms of underground methane-rich coal seams in the revised model.The validation and superiority of the new model were demonstrated against previous results. The model was applied to quantify the self-heating susceptibilities associated with methane desorption diffusion time, coal permeability, differential pressure of leakage, coal-oxidation reaction heat and coal-oxidation rate. The simulation results show that (1) The compositional gas flow in fractures is related not only with coal permeability and methane desorption rate, but also with gas thermal expansion induced self-acceleration effect. (2) The desorption methane in coal matrix infiltration into the fractures can not only dilute oxygen concentration and inert combustible coal, but also hinder outside air leakage into porous coal media, even promote the coal self-heating at the later stage due to gas self-accelerating effect and matrix shrinkage. (3) The larger differential pressure of leakage, coal permeability, coal-oxidation reaction heat and pre-exponential factor can enhance coal self-heating, whereas the larger coal-oxidation activation energy may retard coal self-heating. The simulation results can be used to better understand the coupling mechanism of coal–gas-heating interactions associated with methane and coal spontaneous combustion, and provide some suggestions as to how to control the variables or parameters to retard or suppress the symbiosis disasters of gas and coal spontaneous combustion in underground porous coal media.

Micromachined rectangular-spiral-coil actuator for radio-controlled cantilever-like actuation

This paper reports of a micromachined shape-memory-alloy (SMA) actuator in the shape of a rectangular spiral coil that is designed to perform cantilever-like actuation. The SMA-coil actuator itself forms a passive resonant circuit that functions as a wireless heat source activated using external radio-frequency (RF) electromagnetic fields for frequency-selective control of the actuation. The SiO2 stress layers are selectively patterned on the actuator structure of nickel–titanium SMA, or Nitinol, to manipulate the cantilever profile at the nominal cold state. RF radiation with varying field frequencies shows strong frequency dependence of wireless heating, actuation displacement, and force generation by microfabricated actuators with resonant frequencies of 170–245MHz. The actuators excited at resonance exhibit maximum out-of-plane displacement and force of 215μm and 71mN, respectively. The developed wireless SMA actuator not only provides >2× the force and 2× more power-efficient response compared to the preceding design with similar self-heating mechanism but also offers broader potential applications brought by its commonly adopted cantilever-based actuation.

Toward Fully‐ and Semi‐passive Operation of a Liquid‐Fed Direct Methanol Fuel Cell

AbstractThis work reports the performance characteristics of a liquid‐fed direct methanol fuel cell (DMFC) operated in both fully‐ and semi‐passive conditions. For the latter case, a blower is used to provide forced air convection at the cathode so as to reveal how and how much a passive DMFC suffers from its structural constraint and also the mass and heat transfer limitations. The results based on the fully passive operation suggest that the cell performance is greatly affected by the level of methanol concentration. In this study, 2 M performs the best when the cell uses different structural setups. Besides, the effects of ambient temperature and the cathode self‐heating mechanisms are also explored under a fully passive condition. For the semi‐passive operation, forced air convection is proved to be helpful in enhancing oxygen delivery but may lead to faster heat and water dissipation and thus significantly reduces the cell performance. An optimal blowing intensity is obtained when the blower operates at a half speed. When the cathode diffusion layer is removed, the effects of active air supply become weakened. Considering the limited performance improvement and parasitic losses caused by a blower, we believe the self‐breathing mode is still an attractive choice.

Self-heating in kinematically complex magnetohydrodynamic flows

The non-modal self-heating mechanism driven by the velocity shear in kinematically complex magnetohydrodynamic (MHD) plasma flows is considered. The study is based on the full set of MHD equations including dissipative terms. The equations are linearized and unstable modes in the flow are looked for. Two different cases are specified and studied: (a) the instability related to an exponential evolution of the wave vector and (b) the parametric instability, which takes place when the components of the wave vector evolve in time periodically. By examining the dissipative terms, it is shown that the self-heating rate provided by viscous damping is of the same order of magnitude as that due to the magnetic resistivity. It is found that the heating efficiency of the exponential instability is higher than that of the parametric instability.

Dispersion effect on the current voltage characteristic of AlGaN/GaN high electron mobility transistors

The current voltage (IV) characteristics are greatly influenced by the dispersion effects in AlGaN/GaN high electron mobility transistors. The direct current (DC) IV and pulsed IV measurements are performed to give a deep investigation into the dispersion effects, which are mainly related to the trap and self-heating mechanisms. The results show that traps play an important role in the kink effects, and high stress can introduce more traps and defects in the device. With the help of the pulsed IV measurements, the trapping effects and self-heating effects can be separated. The impact of time constants on the dispersion effects is also discussed. In order to achieve an accurate static DC IV measurement, the steady state of the bias points must be considered carefully to avoid the dispersion effects.

Modeling of Self-Heating Mechanism in the Design of Superconducting Limiters

We propose a modeling method to simulate how the local temperature rises, due to power dissipation, and how this affects the performance of a high temperature superconductor (HTS) limiter. For given material properties, power, and frequency we determined, by use of both electromagnetic and thermal modeling, the spatial distribution of the temperature rise across an HTS transmission line. This temperature rise in turn affects the local description of the superconductor nonlinearities. To model this effect, we use an iterative technique that combines the Weeks-Sheen method to calculate the current-density distribution with a finite-element method to calculate temperature rise at each point of the transmission line. Simulations of coplanar waveguide HTS limiters on sapphire and quartz are presented.

Degradation of Metal-Induced Laterally Crystallized n-Type Polycrystalline Silicon Thin-Film Transistors Under Synchronized Voltage Stress

Device degradation of n-type metal-induced laterally crystallized polycrystalline silicon thin-film transistors is systematically investigated under synchronized Vg and Vd pulse stresses. ON-state degradation is dominated by a pulse duty-time-related self-heating (SH) mechanism for low-frequency stresses whereas by a pulse transient time-related dynamic hot carrier (HC) mechanism for high-frequency stresses. OFF-state degradation is dominated by the dynamic HC effect, irrespective of stress frequency. It is first observed that such dynamic HC degradation is independent of the pulse falling time (tf) but dependent on the rising time (tr). During tr, HCs are generated in the drain depletion region by a high transient coupling electric field arising from Vg switching. However, during tf, the HC effect is screened by SH that caused high temperature rise. Device saturation is confirmed to play a key role in dynamic HC degradation under synchronized stresses. The proposed degradation model is verified by comparing it with various stress test results.

Nonmodal Cascade in the Compressible Solar Atmosphere: Self-Heating, an Alternative Way to Enhance Wave Heating

The nonmodal self-heating mechanism recently proposed by Rogava is applied to the medium of solar coronal holes with an inhomogeneous plasma flow along the magnetic field lines. The viscosity force is assumed to be anisotropic. The efficiency of the nonmodal cascade process is examined for different sets of environmental parameters and for different wave parameters. It is concluded that the proposed mechanism can serve as an alternative mechanism for explaining the significant heat production in the lower corona, even when only laminar values of the viscosity coefficients are taken into account.

Analysis and modeling of the self-heating effect in SiGe HBTs

This paper investigates the self-heating effect in SiGe heterojunction bipolar transistors. A physical study leads to a nonlinear physical model describing static and dynamic self-heating mechanism. The implementation of this model using an electrical equivalent circuit is presented. Our approach is validated using measurements on devices from different technologies. System configuration, measurement, and calibration issues are presented.

IGBT dynamics for clamped inductive switching

Clamped inductive switching performance of insulated gate bipolar transistors (IGBTs) have been studied in detail with the aid of extensive measurements and numerical simulations. Internal dynamics of a latch-up free punch-through IGBT during clamped inductive switching is studied using two-dimensional (2-D) mixed device and circuit simulations incorporating the self-heating mechanism. Failure of IGBT during inductive load turn-off is shown to occur due to thermally assisted carrier multiplication at the reverse biased p-base n-drift region junction under the emitter contact.